Heat a large skillet over medium heat and add the extra-virgin olive oil.
Add the thinly sliced onions and sliced green pepper to the skillet. Season with salt and pepper and cook, stirring occasionally, until the onions are soft and caramelized, about 7–10 minutes. Remove from heat and set aside.
Preheat a grill or grill pan to medium-high heat.
Season the sirloin steak on both sides with garlic salt and black pepper. Grill the steak about 2 minutes per side, or until it reaches your desired doneness. Remove from heat and let rest briefly.
Slice the rested steak thinly against the grain.
Assemble the sandwiches by placing sliced steak and the caramelized onion-and-pepper mixture into each toasted hoagie bun, then top evenly with provolone slices.
Broil briefly until the cheese is fully melted, or simply close the sandwich and allow the residual heat from the meat to melt the cheese.
Serve with Mezzetta jalapeños and peperoncini on the side.